1. Field of the Invention
The present invention relates to differential pressure sensors.
2. Prior Art
In the prior art there are various differential pressure sensors which use isolation diaphragms that permit the sensing units to be isolated from corrosive fluids that might be measured.
U.S. Pat. No. 3,618,390 issued to Roger L. Frick, illustrates a capacitance type differential pressure sensor that has isolation diaphragms, and wherein the pressure sensed by the isolation diaphragms is transmitted to the sensing diaphragm through an oil filling. In the sensor shown in U.S. Pat. No. 3,618,390, there is a substantial "head" of oil, and where the unit is subjected to acceleration forces, the accuracy can be adversely affected. However, the advantages of utilizing isolation diaphragms and an incompressible fluid which transmits the pressures from the isolation diaphragms to the sensing diaphragm are disclosed in this patent.
Other types of differential pressure sensors are shown in the prior art, for example German Publication No. 2,164,896 shows a differential pressure capsule similar to that shown in U.S. Pat. No. 3,618,390 except that it uses an electromagnetic or inductance type transducing element. The device shown in this German Publication does have isolation diaphragms, and apparently uses an oil filling to transmit pressure from the isolation diaphragms to the sensing unit.
British Pat. No. 1,335,873 also shows a differential pressure transducer having a variable reluctance transducing element, and showing annular isolation diaphragms, with an oil filling that transfers the pressure from the isolation diaphragms to the sensing assembly. In this particular British patent, corrugated central diaphragms are used to compensate for oil volume changes under differing temperatures.
U.S. Pat. No. 3,894,436 has annular corrugated flexible diaphragms and a centrally mounted diaphragm in the same plane as the annular diaphragms and with oil filling between opposite sides of a differential pressure cell. The central diaphragms in this device are the measuring diaphragms, and the annular outer diaphragms are present to take up volume changes in the oil due to temperature changes.
An electromagnetic differential pressure transducer which has an annular corrugated diaphragm is shown in U.S. Pat. Nos. 3,680,387, and 3,882,443 also shows a differential pressure transducer that utilizes oil filling, and isolation diaphragms acting through the oil filling to displace the sensing element. Temperature compensating diaphragms are also utilized in this device to absorb expansion and contraction of the oil or liquid used in the sensor.